US2483226A - Electronic noise generator - Google Patents

Description

Sept. 27, 1949. E. B. NEWMAN ELECTRONIC NoIsE GENERATOR Filed Oct. 29, 1945 l I I I I I I I I I I I I I I I I I l I I l I l l I I I I l I I I I Patented Sept. 27, 1949 #uN/Iran TAT-Es Parleur oFFlcE ELECTRONIC NOISE GENERATOR 'Edwin B. Newman, vCambridge Massi, assigner -t`o the United States of America as represented by the Executive 'Secretary 'of the lO'flce fof v'Scientific Research and Development Application october v29, 1945, "Serial Na 6255163' `6 Claims. (Cl. 25W-36') VThe invention 'described herein may be manufactured and used by or for the Government for governmental purposes without payment to me of any royalty thereon.

This hinvention relates to -noise generators and more especially to 'electronic generation of airplane noise.

"The noise which vis encountered -in aircraft has two pfinci'palcomponents. The yiirst is that of the propeller which sends out radially compression Waves of very vhigh intensity, producing a characteristic'throbbingior humming. This noise canberepresented as a discontinuousor line spectruir'i, vconsisting -of ya 'fundamental `frequency equal 'to the frequency with which the propeller tips pass any given point, "and a series of higher frequencies which are 'harmonics or multiples of the fundamental frequencies. The secondsource oi noise turbulence inthe air stream, and such turbulence is to -be found associated with various irregularities presented by' an air ship to the smooth Tl'o'tv passing its surface. This noise consists 'of a continuous or random spectrum. In other Words, A-this noise Ycontains all frequencies from the lowest to `the highest audible sounds. Thereare also l'other sources of noise which may bedisre'gafrd'ed for the purposes 'of this invention.

It 1'is an object of the invention to provide 1an apparatus `for the electronic generation of noise, and to synthesize 4el'eetrical'ly a discontinuous vor line spectrum, and a continuous or random spectruth. n'oth'e'r 'ob-'ject of the yin'vei'ition is toisolate adesired bandldf `frequencies fr'omfone of the spetia an'd'to mik the'-des`relf bar-1d of frequencies with "frequencies in the other of thelspectra, to'produceas nearly as possible the true 'principal coniphrl'itslo "lplanelnoises Another object of the invention 'is to modify electrical :devices used ih'pibauln'g "a ll'hefpectiiinahd a ladm lSpectrum in a manner intended to facilitate the application of these devices to generation of airp'iitle 'II'SGL the accompanying drawings:

Fig. 2 is a wiring diagram of noise generating apparatus.

There are two fundamental devices for electronically vr'roducing noises. The rst of these is the relaxation oscillator. The oscillator consists of La condenser Awhich is charged at a gradually decreasing 'rate through ahigh resistance. When the poten'ti'al'across this condenser reaches a critical high value, Lthe gas 'containedfi'n theftube is ionized and the condenser is rapidlydischarg'ed.

When the' potentialihasfallen to a-'second "critical low valuethe current 'throughthe tube 4no longer maintains ionizationand the tube ceases to conduct. A harmonic analysis of this'wave form reveal'sthe ffact that it contains a fundamental frequency corresponding with pericd :and a long series Iof higher harmonics. This fundamental circuit is used to provide line spectrum 'generators.

'There are vseveral methods yof generating the random noise spectrum, depending -upon the behav-iorfof electrons or of ions. 'The `most useful sou-roe of random potentials' is 'a igasefilled diode. In 'this-case, current Tis carried `*principally by the krelatively large andsloivm'oving ions of gas. Be-

cause' of their 'propertiesrthe momentary uetuation'sin the 'cathode-toanode current of such a gas diode "tl 'Bftied thS'e plfduced by other sources. 1t iis to loe'.anticipatedthat the maximum output Will bepr-'odueed W'liensuch va ytube isop-erating near the criticallyiomzation point. VInapplying such potentials top'urposes of this invention, it is necessary first to isolate and amplify this noise, andthen to pass it through such ltering networks as will give 'the proper distribution of 'energy Vin the various regions of the sound spectrum.

In accordance with the present invention, the two types of sound generating equipment, i. e., relaxation oscillator and the -gas-lled diode, are combined on one chassis, as illustrated in the block diagram-of Fig, Y1,an'd the wiring diagram of Fig. 2.

Considering 'the Wiring diagram more in detail, the 2051 tube is the generator of the continuous spectrum. The output of this tube -is fed through a tube GSJ'I to the No. 1 g'ridof a 1.612 mixer -amplier tube. At the same time the line spectrum is Vgenera-'ted in the 8'84 tube, the output of vhich is fed to the No. `3 grid of Ythe 1612 mixer ube.

The output 'of the mixer tube -is lead to -a second 6ST? tube, triode 'connected, in the grid circuit oi which is connected the frequency-control network. The latter network is made up of resistors R-13, R-14, andthe condenser C-18. The power supply u sesa 5Zk4'tube, the output -of which is stabilized with two VR--30 voltage-regulator tubes connected in series.

The amplifier fand power supply components of this unit may -b'e modied Without substantially `affecting the operation Lvof the noise generator. Some Valueslmay loe 'changed without changing the operation of v1 thedevice. For insta-nce, the 25henry choke coil in the plate lead to the 884 tube should be fairly close to this rated value. In the same way, the values of the three condensers, C-13, C-16, and C-17, and the resistors R-20 and R-24, in the network associated with this tube, should not be changed. Other values which should be adhered to fairly closely are the resistors R-13 and R- 14 and the condenser C-18 which form the frequency control network in the grid circuit of the last tube. R-25 may normally be omitted. However, it may be included in case it is desired to increase the high frequency output of the generator. Changes in other circuit values should be consistent with good engineering practice.

In making up the circuit for the line spectrum generator, it has been found advantageous to utilize current flowing into and out of the condenser C-13 for generating the desired potential. This purpose has been accomplished by inserting a resistor R-ZO in the lead from the condenser to the ground, and taking olf the potential across this resistor. 1t has been found that more of the energy from such a generator will be found in the lower frequencies if the rate of discharge ."s slowed down somewhat. For this purpose, a large inductance Li, as shown in Fig. 2, has been inserted in the lead of the gas discharge tube. It is desirable to stabilize the frequency of the line spectrum generator, and this object is accomplished by using a triode 884 in place of a diode and applying a small alternating current potential to the grid through R-5. It has also been found essential to provide a band of frequencies selected from the lower harmonics of the output of the line spectrum generator. This important objective has been attained by passing the generator output through filters as shown in Fig. 2, which give a characteristic band made up of components between 125 cycles and 250 cycles. One would consist of C17 and R-24; the other would be made up of the sloping network consisting of R13, R14, and Cia.

The following list of components indicate values that have been found to provide the desirable noise reproduction although these values may be changed to alter the resultant noise without departing from the spirit and scope of this invention.

RlRcRaR-Rs 50,000 ohms 1w RsR'zRaRe 100,000 ohms 1w RioRii 1,000 ohms 1w R12R13Ri4 500,000 Ohms 1/gw R l megohm 1w RmRnRis 250,000 ohms 1w R19 2 megohm 1w R20 10,000 ohms 1w R21R22 5,000 Ohms 1w 12.2312241325 0.5 megohm potentiometer R25 2,000 ohms 10a R27 4,000 ohms 10e adjustable L1 25 henry choke T1 Power transformer S1. Swith C1C2C3C4C5C6 40 afd., electrolytic, 450 V.

working C'ICBCQ 0.1 Iafd., paper CioCiiClz 20 afd., electrolytic, 25 V.

working C13 .01 afd., paper C14C1s .25 afd., paper C16 .05 afd., paper C17 .003 afd., paper C18 .005 afd., paper G19 .5 Aafd., paper The most important features of this circuit arrangement are found in connection with the inductance element used with the line spectrum generator, and the filter elements used to produce a selective band of frequencies of the line spectrum generator. The inductance element is particularly effective in generating low frequency components of airplane noise which are sometimes difficult to produce in proper values. The frequency control network is capable of emphasizing a particular group of components which have been found to closely simulate similar components occurring in average airplane noises. This particular band of frequencies from the line spectrum generator, as already disclosed, has been found to occur between 125 and 250 cycles. Determination of this band of frequencies is an important element in the method of modifying the line spectrum to simulate a similar component in airplane noise. A further point is to modify the output of the random generator so as to decrease the intensity of frequencies above 250 cycles, at the rate of a frequency range of 5-7 decibels per octave.

Any good power amplifier with a Wide range of frequency response and a reasonably small distortion will adequately meet the needs of this noise generator. It should have good dynamic power handling capacity. The power needed will depend principally upon the sound level to be achieved, and upon the characteristics of the room in which the noise is to be produced. For example, a battery of six 50-watt power amplifiers is sufficient to produce an overall noise level of better than db in a room of 10,000 cu. ft. volume, with hard plaster walls. For most purposes, commercially available 50-watt power amplifiers are quite suitable.

If more than 50-watt power is required, these units can be used in parallel with suitable adjustment of the output circuit to match the impedance of the speakers used. Loudspeaking equipment of the type particularly utilized in systems for theatre application have been found to give best results. Current engineering practice indicates the desirability of using a dividing network and both a low frequency and a high frequency horn. Satisfactory results have been obtained from a system which includes a Jensen PLJ18B in a brass reflex cabinet to handle frequencies up to 400 cycles. This apparatus was set up in a brick-walled room of 2500 cu. ft. volume.

The noise generating equipment described is effective in synthesizing an average noise generally corresponding to a noise field in which the pilot of a plane is required to operate. By simulating a noise field by means of electronic generation, more realistic training conditions are available to student pilots and others. This is especially useful in determining the ability of the pilot to hear and understand directions received in the earphones or other types of communication equipment. Electronically generated noise can also be made use of in various other training situations, covering a variety of types of equipment.

Having thus described my invention, what I claim is:

l. A device for electrically simulating the noise of an airplane in flight over a sound producing means comprising a random spectrum generator for producing oscillations corresponding to the oscillations of noise produced by air turbulence about the airplanes structure in the airstream, a line spectrum generator having an inductance means in its output circuit effective in generating low frequency components of airplane noise wherein said line spectrum generator produces a fundamental frequency and harmonics thereof corresponding to the frequencies produced by noise from the propeller, means to combine the output oscillations of both spectrum generators in mixer amplifier circuit, means coupling the mixer amplifier output to a network having an adjustable resistance included therein for adjusting the frequency transmission characteristic, and an output power amplifier coupled to said network whereby the mixed oscillations are amplified for sound production.

2. An electron device for simulating the noises produced by an airplane in flight comprising a relaxation oscillator in a circuit having an inductance in its output effective in generating low frequency -components of airplane noise wherein said relaxation cscillator produces voltages of a fundamental frequency and harmonics thereof corresponding in frequencies to those of propeller noise, a gas-filled tube for producing voltages of random frequencies corresponding to frequencies produced by the noise of air turbulence of the airplane structure in the air stream, means connecting the output voltages of said relaxation oscillator and said gas-lled tube to a mixer amplifier tube, a network having a fixed resistance coupled to a second fixed resistance arranged in parallel with a variable resistance and a condenser wherein the Variable resistance is adjustable to control the frequency transmission characteristics to emphasize particular frequency components closely simulating frequency components of noise of airplanes, means coupling the output of said mixer amplifier tube with said network, and means for connecting said network with a wide frequency range sound producing system for converting the mixed oscillations of said relaxation oscillator and said gas-filled tube into sound of proper tone and volume to simulate airplane noises in iiight.

3. An electronic device for generating a plurality of audio frequencies corresponding to the audio frequencies in the noise produced by aircraft in flight comprising, a relaxation oscillator circuit having an inductance means in its output effective in slowing down the rate of discharge to generate low frequency components of airplane noise wherein said relaxation oscillator produces voltages of a fundamental frequency and harmonics thereof corresponding to frequencies of propeller noise, a gas discharge tube circuit for producing voltages of random frequencies corresponding to frequencies of noise produced by air turbulence of the airplane structure in the air stream, the output voltages of said relaxation oscillator circuit and the output Voltages of said gas discharge tube circuit being connected to separate control grids of a mixer ampliner tube, means connecting the output of said mixer amplifier tube to a network having a fixed resistance coupled in series to a second Xed resistance arranged in parallel with a variable resistance and a condenser wherein the variable resistance is adjustable to control the frequency transmission characteristic to emphasize particular frequency components closely simulating frequency components of noise of airplanes, means connecting the output of said network through a pre-amplifier means to a power amplifier means, and means for connecting said power amplifier to a wide frequency range sound producing system whereby the propeller and air turbulence noises produced by an airplane in flight may be simulated in tone and sound volume.

4. An electronic device as set forth in claim 3 wherein the output of the relaxation oscillator circuit includes lters to give a characteristic frequency band made up of components between cycles and 250 cycles,

5. An apparatus as set out in claim 4 in which said relaxation oscillator circuit comprises a condenser, means providing a charging circuit for said condenser, a resistance in said charging circuit across which said fundamental harmcnic frequencies are produced, means providing a discharge circuit for said condenser comprising a gas-filled tube acting to discharge said condenser when the condenser voltage reaches a predetermined value, and inductance means in series with the anode of said gas-filled tube for modifying the discharge characteristic of said v condenser.

6. An apparatus as set forth in claim 5 wherein f the gas discharge tube of said gas discharge tube circuit is connected in series with a resistance across a source of direct Voltage whereby said random frequency voltages are developed across said resistance.

EDWIN B. NEWMAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,994,902 Trouant Mar. 19, 1935 2,097,066 Hoover Oct. 26, 1937 2,121,142 Dudley June 21, 1938 2,139,023 Kock Dec. 6, 1938 2,165,509 Ring et al July l1, 1939 2,183,248 RiesZ Dec. 12, 1939 2,194,298 Dudley Mar. 19, 1940 2,207,620 Hilferty Juiy 9, 1940 2,253,975 Guanella Aug. 26, 1941 2,353,499 Furington July 1l, 1944 2,354,699 Owens Aug. 1, 1944 2,373,560 Hanert Apr. 10, 1945 2,416,307 Grieg Feb. 25, 1947 2,416,327 Labin Feb. 25, 1947 2,445,233 Montgomery July 13, 1948

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